Method for fluidized low-temperature carbonization of coal



Dec. 12, 1950 K. J. NELSON 2,534,051

METHOD FOR FLUIDIZED LOW-TEMPERATURE CARBONIZATION 0F COAL Filed Nov.22, 1946 VAPOR PRODUCTS 7/07- emu:- REC 11c. ULAT/OM El. awn-aw.

'RAW COAL FEED INLET n W m c OJ cm H 2 x 7 LB J C of carbonaceoussolids.

Patented Dec. 12, 1950 METHOD FOR FLUIDIZED LOW-TEMPERA- TUBECARBONIZATION F COAL Karl J. Nelson, Cranford, N. J assignor to StandardOil Development Company, a corporation of Delaware Application November22, 1946, Serial No. 711,755

4 Claims. 1

The present invention relates to the handling More particularly, theinvention is concerned with the treatment of finely divided carbonaceoussolids such as all types of coal, ,brown coal, lignite, oil shale, tarsands, asphalt, cellulosic materials including lignin, etc., employingthe so-called fluid solids technique.

Heretofore, solid carbonaceous materials of the type mentioned abovehave been normally treated at elevated temperatures in fixed bedoperation to form liquid and gaseous fuels such as light oils, tars,coal gas, producer gas and water gas. However, these processes involveeither discontinuous operation or inefficient conversion of theavailable carbon into heat and volatile fuels.

The processes may be made fully continuous by employing the fluid solidstechnique in which the reactions take place in a dense fluidized bed offinely divided solids maintained in a turbulent ebullient state by meansof fluidizing gases. This technique has highly desirable additionaladvantages including greatly improved heat distribu-- tion andefficiency of solids utilization.

However, difficulties have been encountered in handling the finelydivided raw material, such as raw coal, as it comes from the cleaningplant. This material normally has a particle size suitable forfiuidization i. e. about 8 mesh by zero or by zero. When this type ofcoal is fed to storage hoppers, mechanical feed hoppers, aeratedstandpipes and the like handling equipment required in fluid solidsoperation considerable plugging, bridging and packing is experienced asa result of the sticky character of the moist coal which containsusually about 240% or more of free Water.

One way of removing the moisture content of the coal is drying with airor other gases at elevated temperatures in conventional dryingequipment. This adds materially to the investment and operating cost. Inaddition, the fine coal particles frequently agglomerate during suchconventional dehydration treatment and have to be reground prior tofluidization. Furthermore, dehydration by mere heating requires acareful temperature control because effective dehydration temperaturesmay approach closely the plasticizing temperature of the coal. Thus, itmay happen that the attempt to remove one cause of plugging createsanother one in the form of a plastic sticky'material.

The present invention overcomes the aforementioned difficulties andaffords various additional advantages. These advantages, the nature ofthe invention and the manner in which it is 2 carried out will be fullyunderstood from the following description thereof read with reference tothe accompanying drawing.

It is, therefore, the principal object of my invention to provideimproved means for handling finely divided carbonaceous solids.

Another object of my invention is to provide means for improving theutility of carbonaceous solids tor fluid solids handling.

A further object of my invention is to provide means for drying finelydivided carbonaceous solids without airecting their particle size.

A more specific object of my invention'is to prepare a dry free-flowingcarbonaceous charge for carbonization and gasification operationsemploying the fluid solids technique.

Other and more specific objects and advantages will appear hereinafter.

In accordance with the presentinvention the moisture content of finelydivided carbonaceous solids is effectively reduced and their stickinessremoved by mixing the mass of the moist carbonaceous solid with amoisture-free or dry moisture-absorbing solid of fiuidizable particlesize having a temperature substantially above the boiling point of waterat the prevailing pressure.

The eiiect oi this procedure is twofold. Mechanically adhering water isevaporated from the moist material by the transfer of sensible heat fromthe "dry" hot material. The dry material absorbs a substantialproportion of water mechanically adhering to the moist material to bringthe average water content of the particles in the solids mixture belowthat causinir stickiness.

The drying effect of the process of my invention is considerably aidedand accelerated when applying fluid solids operation to the mixing offresh feed and dry hot material by utilizing the ideal heat transfer andmixing speed in a turbulent ebullient mass of fluidized solids. In thiscase, relatively short contact times between moist and dry solids, ofsay about 0.1 to 10 minutes are sufllcient to accomplish the desiredeffect.

My preferred dry high temperature solid is low temperature coke or charwithdrawn from the fluidized solids bed of a low temperaturecarbonization process employing the fiuid solids technique attemperatures of about 800-1200 F. This material may be added to themoist fluidized carbonaceous solids substantially at the carbonizationtemperature without further treatment. For instance, when using a moistcoal of fluidizable particle size containing 5% free moisture at 60 F.,about 20-100% by weight of char of a similar particle size having atemperature of about 800-1200 F. or about 60% by weight of char having atemperature of about 850 F. is sumcient to establish a solids mixturehaving an average temperature of about 250 F. and being free-flowing.

When using char from the carbonization process substantially at thecarbonization temperature as the dry solid, the heat balance of thesystem remains substantially unchanged. Furthermore, an externalpremixing step of this type leads to a more rapid uniform distributionof the fresh feed in the fluidized solids bed of the carbonizer, becausea relatively high degree of premixing of fresh feed with dry nonplasticmaterial having the average composition of the material undergoingcarbonization has been accomplished prior to the entry of the fresh feedinto the carbonization zone. This may permit a higher fresh feed rate tothe carbonizer without the danger of plugging or agglomeration of thefresh feed while passing through the plasticization temperature rangewhich normally lies between about 700 and Other suitable, though lessdesirable, dry solids include externally heated solid adsorbents .suchas clay, siliceous gels, iron oxides, etc. of

fluidizable particle size.

My invention affords greatest advantages when applied to a system fortreating fluidized carbonaceous solids wherein the treating conditionsof temperature and residence time are maintained constant by maintaininga constant heat supply while controlling the fresh solids feed as afunction of the treating temperature and the withdrawal of treatedsolids as a function of bed height, i. e. residence time as disclosed inmy copending application Serial No. 716,408, filed December 14, 1946,for improvements in Contacting Solids and Fluids." A system of this typerequires highest mobility of the solids feed as it is accomplished bythe present invention.

Having set forth the general nature and objects, the invention will bebest understood from the subsequent more detailed description in whichreference will be made to the accompanying drawing which shows asemi-diagrammatic illustration of equipment particularly adapted tocarry out a preferred embodiment of my invention.

Referring now to the drawing, the system shown therein essentiallycomprises a fluidized feed standpipe 5 and a fluid solids treatingchamber 20, the functions and cooperation of which will be presentlyexplained. In the following detailed description reference will be madeto the carbonization in chamber 20 of coal supplied through standpipe5." It should be understood, however, that other carbonaceous materialsand other treatments may be employed in the same or similar systems in asubstantially analogous manner.

When applied to the carbonization of a bituminous carbonization coalhaving a free moisture content of about 2 to and a softening point ofabout 700 to 800 F. the raw coal having a fluidizable particle size ofless than 8 mesh, preferably about 50 to 200 mesh is supplied atatmospheric temperature, say about 60 F., to line I by any conventionalconveying means. The finely divided raw coal enters line 3 wherein itmeets hot dry coke or char supplied from carbonization chamber 20 aswill become apparent hereinafter.

The mixture of raw moist coal and hot dry char drops into feed standpipe5. Small amounts of a preferably dry fiuidizing gas such as dry steam,flue gas, nitrogen, make gas, or the like is supplied through line I tothe bottom of standpipe 5 at a superficial velocity of about 0.2-10 ft.per sec-- ond, preferably 0.3 to 2 ft. per second to convert the solidsin standpipe 5 into a dense ebullient mass of fluidized solids having awell defined upper level Lo and exerting a pseudo-hydrostatic pressureon the base of standpipe 5. The apparent density of the fluidized massin standpipe 5 may vary between about 10 and 50 lbs. per cu. ft.

The relative amount and temperature of the char are so controlled thatthe solids mixture in standpipe 5 assumes a temperature of about 150 to300 F. and is kept in a free-flowing state. Depending on the abovementioned moisture content of the coal, about 0.4-0.9 lb. of char havinga temperature of about 850 F. is sumcient for this purpose per lb. offeed coal. It may be desirable to preheat the fluidizing gas suppliedthrough line I to a temperature of about 200-600 F. by a suitable heatexchange with volatile and/or solid carbonization products in anyconventiona1 manner.

A free-flowin preheated fluidized mass of raw coal and char flows underthe pseudo-hydrostatic pressure of standpipe 5 through a control valve 8into carbonization chamber 20. An oxidizing gas such as air and/oroxygen is supplied from line H by compressor I3 via a knock-out andsurge drum l5 and a flow controller ii through line IE to the lowerconical portion of carbonizer 20.

The oxidizing gas enters carbonization zone 23 through a distributinggrid 21. The amount of oxygen supplied should be suflicient to cause alimited combustion of coal constituents adequate to generate at leastthe major proportion of the heat required for carbonization in zone 23.About 0.1 to 0.8 lb. of air or a corresponding amount of oxygen per lb.of raw coal is generally sufficient to establish carbonizationtemperatures of about 800-1400 F. within zone 23.

The finely divided coal and coke in zone 23 are fluidized by the fluegases and volatile carbonization products to form a dense turbulent massof solids resembling a boiling liquid forming a well defined upper levelL20. Linear gas velocities within the approximate limits of about 0.3 to10 ft. per second, preferably 0.3-3 ft. per second are generallysuitable to establish a fluidized mass of an apparent density of about10-60 lbs. per cu. ft. in zone 23 at the particle sizes here involved.

Volatile carbonization products such as coal gas, light oils, tar, etc.pass overhead from level ho through a conventional gas-solids separatorsuch as cyclone 25. carbonaceous solids fines separated in cyclone 25may be returned to zone 23 through pipe 21. Product vapors and gasessubstantially free of solids are passed through line 29 to aconventional product recovery system (not shown).

Carbonized solids consisting essentially of dry hot char are withdrawndownwardly under the pseudo-hydrostatic pressure of the fluidized bed inzone 23 through a bottom drawoff 3| which may be aerated through one ormore taps 33. The speed of solids withdrawal through pipe 3| isregulated by a. slide valve 35. Dry char substantially at thetemperature of carbonization zone 23 drops into a conveying means suchas a screw conveyor 31 from which it may be passed to cooling means andstorage (not shown) A substantial portion'of the hot dry char, which mayamount to about 20 to by weight of fresh coal to be carbonized isdiverted from conveyor 31 to a suitable pick-up means such as elevator39 and hoisted above the upper end of standpipe 5. This hot char thenfiows into line 3 and from there to standpipe 5 to perform the functionsof drying and preheating the raw coal as described above.

In accordance with the preferred modification of this embodiment of myinvention, the solids flow through standpipe 5 is controlled bytemperature responsive means such as thermocouple ll arranged in zone 23and the solids flow through pipe. 3| is regulated by level responsivemeans 43 actuated by level L20 while the supply of oxidizing gas throughline [9 is maintained substantially constant, as described in greaterdetail in my copending application mentioned above. In this manner, aperfect automatic control of the operating conditions may beaccomplished, no irregularities in the raw coal feed being encountered.

It will be understood that the heat required for carbonization may besupplied by other means than partial combustion within zone 23. Forexample, the sensible heat of a heating gas heated above carbonizationtemperature in a separate heater may be employed for this purpose. Heatmay also be generated by burning char withdrawn from zone 23 in aseparate fluid combustion zone and recirculating hot solid combustionresidue to zone 23. Details of these heating methods are disclosed in mycopending application identified above.

The process of my present invention may be applied with greatestadvantage to a system of the type illustrated in the drawing. Othermeans of controlling the operating conditions and of conveying thesubdivided solids may be used without deviating from the spirit of theinvention.

Chamber 20 may serve as a coal preheating or gasification zone by asuitable change of the treating temperatures and the gas suppliedthrough line I9. Preheating may be carried out by merely lowering thetemperature within zone 23 to a level below incipient carbonization andcoal softening. Water gas may be produced by raising the temperature ofzone 23 to about 1500 F.-2500 F. and introducing an amount of steamthrough line I! sufilcient to support the desired conversion of carboninto CO and H2. Other modifications of my invention will occur to thoseskilled in the art.

My invention will be further illustrated by the following specificexample.

Example Operating conditions for the carbonization of Pittsburghseam-bituminous coal containing 5% free moisture without plugging of thecarbonizer and feeding device in a system of the type illustrated in thedrawing may be chosen as given elow:

Carbonizer temperature F 900 Raw coal rate to feed standpipe -lbs./hr2000 Raw coal temperature F 60 Raw coal particle size:

On 8 mesh per cent 0.2 On 14 mesh do 22.2 On 48 mesh do 77.2 On mesh do88.8 On 200 mesh do 96.2 Through 200 mesh do 3.8 Air to carbonizer (atF.) S. C. F. M

Superficial gas velocity in carbonizer ft./sec-- 3 6 Solidsconcentration in carbonizer bed lbs./cu. ft Char withdrawn throughbottom drawoff lbs./hr 2640 The foregoing description and exemplary operations have served to illustrate specific applications and results ofmy invention. However, other modifications obvious to those skilled inthe art are within the scope of my invention. Only such limitationsshould be imposed on the invention as are indicated in the appendedclaims.

I claim:

1. In the low temperature carbonization of subdivided bituminous coal atcarbonization temperatures of about 8001200 F. in the form of a denseturbulent fluidized mass maintained at said carbonization temperaturesin a carbonization zone by burning a portion of said coal in saidcarbonization zone, the improvement which comprises maintaining thebituminous coal to be carbonized as a dense turbulent fluidized mass ofsolids having a fluidizable particle size in a drying zone at a dryingtemperature, withdrawing substantially dry moisture-absorbingfiuidizable char having a particle size similar to that of saidbituminous coal downwardly from said carbonization zone, adding saidwithdrawn char to said drying zone substantially at said carbonizationtemperature, said added char supplying in the form of sensible heat ofsolids substantially all the heat required for drying, and supplying afluidized mixture of bituminous coal and char from said drying zone tosaid carbonization zone.

2. The process of claim 1 wherein said mixture is supplied from saiddrying zone to said carbonization zone under the pseudo-hydrostaticpressure of said fluidized mass in said drying zone.

3. The process of claim 1 wherein the temperature of said char added tosaid drying zone is about 800-1200 F., the amount of dry char added tosaid drying zone is about 20-100% by weight of said subdivided bitminouscoal, and the time of contact between the solids charged to said dryingzone is about 0.5 to 60 minutes.

4. The process of claim 1 wherein the supply of solids from said dryingzone to said carbonization zone is controlled by the temperature of saidcarbonization zone.

KARL J. NELSON.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

1. IN THE LOW TEMPERATURE CARBONIZATION OF SUBDIVIDED BITUMINOUS COAL ATCARBONIZATION TEMPERATURES OF ABOUT 800 DEGREES - 1200 DEGREES F IN THEFORM OF A DENSE TURBULENT FLUIDIZED MASS MAINTAINED AT SAID CARBONIZATONTEMPERATURES IN A CARBONIZATION ZONE BY BURNING A PORTION OF SAID COALIN SAID CARBONIZATION ZONE, THE IMPROVEMENT WHICH COMPRISES MAINTAININGTHE BITUMINOUS COAL TO BE CARBONIZED AS A DENSE TURBULENT FLUIDIZED MASSOF SOLIDS HAVING A FLUIDIZABLE PARTICLE SIZE IN A DRYING ZONE AT ADRYING TEMPERATURE, WITHDRAWING SUBSTANTIALLY DRY MOISTURE-ABSORBINGFLUIDIZABLE CHAR HAVING A PARTICLE SIZE SIMILAR TO THAT OF SAID